1. Field of Invention
The invention relates to a method for fabricating a semiconductor-based light-emitting device, such as a light-emitting diode (LED).
2. Description of Related Art
Semiconductor-based light-emitting devices, such as LEDs, often radiate light in a more efficient manner than conventional incandescent light sources, as well as fluorescent light sources. The relatively high power efficiency combined with high brightness associated with LEDs has created substantial interest to displace conventional light sources in a variety of lighting applications.
Typically, a semiconductor-based light-emitting device is formed of multiple layers on a substrate, wherein the various materials and thicknesses thereof are selected to determine the wavelength(s) of light emitted by the device. The chemical composition of each layer is selected in an effort to isolate injected electrical charge carriers into regions (often referred to as quantum wells) for relatively efficient conversion to optical power. Generally, layers on a first side of the junction where a quantum well is formed are doped with donor atoms that result in high electron concentration (such layers are commonly referred to as n-type layers), and layers on a second side of the junction are doped with acceptor atoms that result in a relatively high hole concentration (such layers are commonly referred to as p-type layers).
Group III-nitride semiconductors, such as GaN for its wide direct bandgap, have demonstrated great promise in the formation of semiconductor-based light-emitting devices. In particular, GaN-based optical devices have found use in light emission within the green-blue region of the visible spectrum.
However, the practical, cost-wise implementation of Group III-nitride semiconductors has demonstrated many shortcomings and challenges, including, among other things, the integration of such materials with conventional substrate fabrication, i.e., silicon. Some of the integration challenges, to name a few, include differences in the coefficient of thermal expansion between light-emitting device stack layers and the substrate, adhesion of the light-emitting device stack layers to the substrate, and lattice mismatch between the light-emitting device stack layers and the substrate. Other challenges include, among other things, the preparation of a low resistance metal-semiconductor contact, i.e., contact between a metal layer and a Group III-nitride semiconductor layer. Metal-semiconductor contacts are known to exhibit poor resistance across their junction due to the Schottky barrier arising from the difference in work function of the metal and semiconductor layers.
As a result, in an effort to cure such issues, techniques are sought for integrating Group III-nitride semiconductors with various substrates, including silicon substrates, and with metal contacts.